The back comes off to take the roll film back. There are 4 retainers that hold the different backs on the camera. You can also get an adapter that allows the graphic roll backs to be used instead of the Mamiya backs.

At the risk of seeming dense, since I have neither a Mamiya Press 23 nor any 2x3 filmholders in my possession, I am assuming that the opening on the right rear side is for said 'normal' 2-1/4 x 3-1/4 filmholders.